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Processes in the Unsaturated Zone by Reliable Soil Water Content

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Processes in the Unsaturated Zone by Reliable Soil Water Content sustainability Article Processes in the Unsaturated Zone by Reliable Soil Water Content Estimation: Indications for Soil Water Management from a Sandy Soil Experimental Field in Central Italy Lucio Di Matteo * , Alessandro Spigarelli and Sofia Ortenzi Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Via Pascoli s.n.c., 06123 Perugia, Italy; [email protected] (A.S.); sofi[email protected] (S.O.) * Correspondence: [email protected]; Tel.: +39-0755-849-694 Abstract: Reliable soil moisture data are essential for achieving sustainable water management. In this framework, the performance of devices to estimate the volumetric moisture content by means dielectric properties of soil/water system is of increasing interest. The present work evaluates the performance of the PR2/6 soil moisture profile probe with implications on the understanding of processes involving the unsaturated zone. The calibration at the laboratory scale and the validation in an experimental field in Central Italy highlight that although the shape of the moisture profile is the same, there are essential differences between soil moisture values obtained by the calibrated equation and those obtained by the manufacturer one. These differences are up to 10 percentage points for fine-grained soils containing iron oxides. Inaccurate estimates of soil moisture content do not help with understanding the soil water dynamic, especially after rainy periods. The sum of antecedent soil moisture conditions (the Antecedent Soil moisture Index (ASI)) and rainfall related to different stormflow can be used to define the threshold value above which the runoff significantly increases. Without an accurate calibration process, the ASI index is overestimated, thereby affecting the threshold evaluation. Further studies on other types of materials and in different climatic Citation: Di Matteo, L.; Spigarelli, A.; conditions are needed to implement an effective monitoring network useful to manage the soil water Ortenzi, S. Processes in the and to support the validation of remote sensing data and hydrological soil models. Unsaturated Zone by Reliable Soil Water Content Estimation: Keywords: soil moisture; PR2/6 probe; infiltration; soil water management Indications for Soil Water Management from a Sandy Soil Experimental Field in Central Italy. Sustainability 2021, 13, 227. 1. Introduction https://doi.org/10.3390/su13010227 In the effort to achieve most of the United Nation (UN) Sustainable Development Received: 12 December 2020 Goals (SDGs), it is necessary to move towards a sustainable use and management of the Accepted: 25 December 2020 soil-water system [1]. For achieving sustainable water management, the monitoring of soil Published: 29 December 2020 moisture content is mandatory in the understanding of processes involving the unsaturated zone (e.g., infiltration, migration of pollutants, etc.) [2]. As reported by Vanclooster [3], Publisher’s Note: MDPI stays neu- the monitoring is linked to the modeling of soil processes and space-based environmental tral with regard to jurisdictional claims monitoring systems can give benefits, such as rapid and effective adaptation of the water in published maps and institutional sector [4]. In this framework, sustainable soil and water management objectives require affiliations. reliable data. According to Escorihuela and Quintana–Seguí [5], reliable soil moisture data are essential in the Mediterranean region because they affect the characteristics of land processes such as droughts and floods. The ongoing climate change affecting this area Copyright: © 2020 by the authors. Li- decreased rainfall during the recharge period (autumn to spring period, e.g., [6–8]) and censee MDPI, Basel, Switzerland. This intensified extreme rainfall events [9]. As recently reported by Mimeau et al. [10], future article is an open access article distributed scenarios indicate a further change of precipitation patterns in the coming years. These under the terms and conditions of the changes are associated with an intensification of extreme rainfall events alternating with Creative Commons Attribution (CC BY) long periods of drought, increasing the pressure on land with progressive degradation license (https://creativecommons.org/ processes affecting soil productivity, irrigation, groundwater recharge, etc. [11]. The moni- licenses/by/4.0/). Sustainability 2021, 13, 227. https://doi.org/10.3390/su13010227 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 227 2 of 15 toring of the soil-water system is crucial in the efficiency of water use, while also improving the agricultural sustainability [12]. Soil moisture can be obtained by direct methods (thermo-gravimetric laboratory method, [13]) or indirect methods, most of which are based on the measurement of the dielectric properties of the soil-water system. The two methods are used locally, have advantages and disadvantages, and are complementary to each other. The gravimetric method is the standard technique to measure the gravimetric water content (θg). It is also useful to calibrate the estimates of the indirect methods. It requires the soil sampling (destructive method), is time-consuming, and cannot be used for the monitoring in the field [14,15]. Indirect methods allow real-time and continuous monitoring of the volumetric water content (θ) and are more and more used in the latest years. Several instruments are available with different accuracy, costs (including the maintenance), the difficulty of installation, and response time [16]. Compared to the Time Domain Reflectometry (TDR), the Capacitance and Frequency Domain Reflectometry (FDR) allow, by means of the profile probe (PP), the estimation of spatial-temporal evolution of θ [17–23]. Moreover, PP can contribute to the implementation of an integrated monitoring system, also aiming to map the spatial variation of the soil water content coupled with other electromagnetic methods, such as the Ground Penetrating Radar, GPR [24,25]. Field investigations also help the calibration and validation of remote sensing data, enhancing the understanding of processes at large scales [26,27]. The quality of locally monitored data is the basis for the re-analysis of the soil water content data from satellites [28]. Although the using of capacitance and FDR profile probes is increasing, the calibration problem and performance of the instrumentations cannot be neglected, especially when they are to be applied in large sites [17,23,29–35]. In other words, the use of manufacturer equations to convert the electromagnetic properties of soils into θ may lead to incorrect assessments of the soil moisture content, with inevitable effects on soil water management and in the understanding of processes occurring in the unsaturated zone [23,36,37]. In this framework, the present work aims to contribute in evaluating the performance of the PR2/6 soil moisture profile probe with implications on devices operating at a frequency of 100 MHz. An experimental field in Central Italy is selected and the monitored data are presented and discussed, taking into account the equation used to estimate the soil water content, the antecedent rainfalls, and the modeled parameters of the unsaturated soil. 2. Materials and Methods 2.1. Experimental Field and Soil Characteristics For the study, an experimental plot 18 m2 wide was selected. The site is located in Central Italy in the left bank of the Tiber River alluvial plain (43◦07033.60” N–12◦26004.76” E, Figure1a), where a fine-grained soil widely outcrops (soil S B). According to the USDA (United States Department of Agriculture) classification, this soil is a loamy-sand deriving from the erosion of marly-calcareous-siliceous rocks (Flysch) widely outcropping in the Tiber River catchment (the mineralogical composition and organic matter content is shown in Table1). The lithological model of the site has been reconstructed with the GPR sur- vey [25], with the Dynamic Probe Super Heavy test (DPSH) and with sampling at different depths (0.15, 0.45, 0.60, 0.80, and 0.95 cm) aiming to obtain the main physical parameters of the soil (Figure1b). The site is equipped by a weather station (rainfall and temperature) managed by the Servizio Idrografico della Regione Umbria (Figure1a) and by a PR2/6 profile probe (Delta-T Devices Ltd., Cambridge, UK), which constitutes an integrated monitoring system. The area is characterized by a mean precipitation of about 860 mm/year and mean annual temperature of 14.5 ◦C (1992–2019 period). As shown in Figure1c, the density of sands tends to increase with depth. Within the in- vestigation range of the infiltration process (in the first-meter depth), there is homogeneous 3 loose sand with dry unit weight (γd) of about 14.33 kN/m and void index (e) of 0.82 (data Sustainability 2021, 13, 227 3 of 15 obtained on the soil samples collected from S6 and S5). These are young alluvial sediments related to the two important floods of the Tiber River occurred in November 2005 and in November–December 2010. The geotechnical characteristics of soil SB, together with some other non-plastic soils (sands and loamy sands of different nature), are shown in Table1. As presented in the introduction section, to check the performance of measurements by indirect methods, the comparison of empirical equations to estimate θ values on similar materials can be useful. The results of this analysis will be presented in Section 3.1. Figure 1. (a) Location of the experimental field; (b) sketch of the survey site with the instruments
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